The present invention relates to support columns, such as concrete support columns for bridges, and, more particularly, to reinforcement of such columns with a composite material.
Support columns, such as bridge supports and supports in parking structures, can occasionally experience forces beyond the forces for which they were designed. This has happened a number of times during earthquakes. The results have been catastrophic, with the collapse of bridges and other structures, loss of life and the loss of use of major highways for many months, and even years. The cost of rebuilding collapsed structures like bridges is so high that sometimes the structures are not rebuilt.
Concrete bridge columns are typically 4 to 8 feet in diameter and 20 to 60 feet high. In an earthquake, the ground shifts not only laterally, but vertically. The lateral shift causes a failure at the column base or in the mid-column because of the inertia of the upper bridge structure being at rest while the lower structure shifts laterally. In the case of the 1994 San Fernando Valley earthquake, the ground also moved up from a thrust fault, which caused the columns to fail in the middle versus the lower sections, where they failed in the 1989 Loma Prieta earthquake.
Pre-1971 bridge columns in California had a sufficient amount of vertical steel, but only had circular reinforcing lap splice bars approximately every 12xe2x80x3. In the 1971 Sylmar earthquake, the 1989 Loma Prieta earthquake, and the 1994 San Fernando Valley earthquake, many of these columns exploded because of the forces, either from the ground moving up in the earthquake, or from the inertia of the bridge deck collapsing down. This caused the columns to explode radially outward in a pear-shaped fashion. More-recently constructed columns have a complete cirumferential reinforcement cage defined by circumferential reinforcing bars spaced approximately on one-inch centers, rather than on the previously used one-foot centers.
Some concrete bridge columns which were already reinforced with embedded steel reinforcing bars have been retrofitted with steel jackets. The steel jackets typically have a thickness between xe2x85x9cxe2x80x3 to 1 inch, depending upon a variety of conditions, including soil conditions, the original design of the column, the height of the column, the amount of load the column carries, etc. In order to retrofit existing columns with additional reinforcement, steel jackets made of semi-cylindrical sections are placed around the outside of the columns, and the sections and jackets are welded together to form adequate confinement. A drawback with the steel jackets is that they must fit as tightly as possible, even though the concrete columns are not always precise in diameter. In order to accomplish this, the columns are individually measured and those measurements are used to fabricate steel jackets of approximately the same diameter. The semi-cylindrical jacket sections are slightly oversized, for example xe2x85x9xe2x80x3 to xc2xcxe2x80x3 oversized in radius. After the jackets are welded in place, they are pumped with an epoxy grout to serve as a medium to transfer from the concrete column to the steel jacket the loads imposed on the column. Sometimes a concrete slurry is injected between the steel jacket and the column, because of the difficulty in fitting the jacket to the column. However, there is shrinkage with the injected concrete and, therefore, there is inadequate load transfer between the column and the jacket. Furthermore, the steel jackets are very heavy and cumbersome to install, even with the aid of power cranes. Moreover, skilled workers, e.g., welders, are required to install the steel jackets, and the jackets are subject to corrosion and require maintenance. Space between the steel jackets and the concrete column is pumped with a pressuring grout to maintain adequate load transfer from the column to the jackets. However, because the column may often be coated with a significant amount of residue and because the steel jacket may have rust on it, the bond between the two load transfer surfaces is often insignificant.
The use of a resin pre-impregnated semi-cured material using carbon fibers or glass fibers or KEVLAR fibers and the use of a wet lay-up system involving high strength fibers and wet resin are currently being pursued. In the wrapping of columns with pre-impregnated tape, an entire machine must be brought to the job site. The use of the machine to wrap the columns can be very difficult in confining situations where the columns are placed very near walls.
Other support columns, which are commonly made of wood, such as utility poles, wharf pilings and bridge supports, occasionally experience exceptional forces, such as in winds or earthquakes. They also suffer from general wear and tear. Furthermore, many wooden utility poles treated with creosote experience dry rot in their lower portions.
By the present invention, apparatus is provided which reinforces support columns to withstand exceptional loads, without having the drawbacks of previously known devices.
A column reinforcement device is provided which is easy to install in the field with unskilled labor. The column reinforcement device can be installed without heavy machinery or heavy tools. The column reinforcement device is premeasured to be the correct diameter, length and thickness for the column. The preformed nature of the device permits it to be precisely premeasured as to width, length and diameter for the particular column on which it is to be used. The appropriate dimensions can be determined as a result of testing in a laboratory. The elimination of the need for calculating, measuring or cutting in the field permits the composite column reinforcement to be installed by an unskilled worker and in severe weather conditions.
The column reinforcement comprises preformed, precured composite members to reinforce the column against failure in earthquakes and other extraordinary events. The device comprises a large plurality of bidirectional continuous, lightweight, high strength, electrically non-conductive nonmetallic fibers extending parallel to one another, and a resinous material encapsulating the fibers.
Because of the uniformity and diameter of bridge column height, the composite column reinforcement members of the present invention are well suited to high volume manufacture and ease of installation in the field with semi-skilled labor.
By the present invention, apparatus is provided which reinforces support columns to withstand exceptional loads, without having the drawbacks of previously known devices.
Concrete is brittle. It has limited ductility.
The composite column reinforcement members according to the present invention greatly increase ductility and confinement for concrete columns. The composite column reinforcement members confine the concrete and prevent the outward expansion or spalling of concrete. If the outward expansion or spalling can be prevented, then the column will be adequate to support the bridge load, even if the concrete is pulverized by the compressive forces from an earthquake.
Because the structural columns are subject to side-to-side loads, not only vertical acceleration and compression, longitudinal fibers are in the reinforcement members to provide an adequate side-to-side load reinforcement.
By encapsulating the concrete columns with the composite reinforcing members, there is nowhere for the concrete to go if the concrete shears or compresses and turns in fact to rubble, because its outer circumference is contained. Consequently, the column and the structure it supports remain intact. In contrast, in an unconfined structure, the acceleration of the column caused by the forces of the earthquake cause the column to either be crushed or to be sheared and the outer portions of concrete to spall off. With this spalling off of concrete, the diameter of column is reduced, its ability to support an upper structure is decreased, and the column fails, along with the upper structure. The composite reinforcing members are intended to be used with concrete columns containing steel reinforcing bars, as well as with columns which do not contain reinforcing bars.
The composite column reinforcement devices can take the form of coil bands having a plurality of concentric elastic convolutions, or jackets defining almost complete cylinders except for a small, axially extending gap.
The composite reinforcement members of the present invention are fixed to concrete columns by a high-elongation adhesive, such as a urethane adhesive, which has an affinity for both the composite and the concrete.
Based on the use of 1,000 psi adhesive, a 61xe2x80x2 diameter column with a 5xe2x80x2 high jacket would have about 13 million pounds of shear strength, thereby making the jacket integral with the concrete column.
The composite column reinforcement members are light enough to be very easily handled by two installers. A 6xe2x80x2 diameter composite reinforcement jacket, 5xe2x80x2 high, and xe2x85x9xe2x80x3 thick weighs approximately 125 pounds. The composite reinforcement members are made relatively thin e.g., so that, if additional reinforcement is required at the base of the column, the middle or the top of the column, additional reinforcement members can simply be placed over the outside of previously applied members, thereby making the system as xe2x80x9capplication friendlyxe2x80x9d and adaptable as possible in the field.
Also, because of the physical flexibility of the composite material, reinforcement members in the form of jackets, or sleeves, can be stacked one inside another, possibly stacking as many as ten jackets in this manner for easy transport with minimal space requirements. With their resilience, the jackets return to their original shape when unpacked at the jobsite.
Because the composite jackets are produced in a factory under controlled conditions, the densities of the filaments and resin is very precise, and the mechanical properties are very uniform, especially compared to filament and resin systems which are laid up wet in the field. The dimensions can be checked, the fiber reinforcement content can be measured through resin burnoff or other laboratory tests, and, therefore, a high degree of uniformity can be obtained.
The skill level required of installers is significantly lower than for steel jackets, which must be welded in place. Basic laborers can apply the composite column reinforcement members of the present invention on freeways, bridges or other structures.
By reinforcing a concrete column with the composite reinforcing members of the present invention, the deformation and the ductility of the column are significantly increased. The present invention confines the concrete from spalling off and increases the ductility of the column during bending by the use of a very simple hand-applied system.
The composite reinforcing jackets rely on the strength of the filaments and their bidirectionality, in that the longitudinal fibers support the column from bending or shearing sideways, while the circumferential fibers support the column from failing in a radial or circumferential direction.